SUMMARY

The wind-sensitive cercal system, well-known for mediating terrestrial
escape responses, may also mediate insect aerial bat-avoidance responses
triggered by wind generated by the approaching bat. One crucial question is
whether enough time exists between detection and capture for the insect to
perform a successful evasive maneuver. A previous study estimated this time to
be 16 ms, based on cockroach behavioral latencies and a prediction for the
detection time derived from a simulated predator moving toward a simulated
prey. However, the detection time may be underestimated since both the
simulated predator and prey lacked certain characteristics present in the
natural situation. In the present study, actual detection times are measured
by recording from wind-sensitive interneurons of a tethered praying mantis
that serves as the target for a flying, attacking bat. Furthermore, using
hot-wire anemometry, we describe and quantify the wind generated by an
attacking bat. Anemometer measurements revealed that the velocity of the
bat-generated wind consistently peaks early with a high acceleration component
(an important parameter for triggering wind-mediated terrestrial responses).
The physiological recordings determined that the mantis cercal system detected
an approaching bat 74 ms before contact, which would provide the insect with
36 ms to perform a maneuver before capture. This should be sufficient time for
the mantis to respond. Although it probably would not have time for a full
response that completely evades the bat, even a partial response might alter
the mantid's trajectory enough to cause the bat to mishandle the insect,
allowing it to escape.

In flying insects possessing cercal systems, another hypothesized role for
the GIs is mediating aerial evasive responses to avoid bat predation,
triggered by detecting wind generated by an approaching bat
(Ganihar et al., 1994).
Although some insects possess auditory systems sensitive to the ultrasonic bat
echolocation calls and perform evasive maneuvers effective in eluding these
predators (Miller and Surlykke,
2001), most nocturnal flying insects lack these specialized active
defenses. However, many of these insects do possess a cercal-GI system and
could potentially benefit from engaging in evasive maneuvers triggered by the
wind generated by an approaching bat.

Ganihar et al. (Ganihar et al.,
1994) described cercal-mediated wind-evoked responses during
tethered flight in the cockroach Periplaneta americana, an insect
that lacks sensitivity to ultrasound. Side-directed wind initiated movements
consistent with turns away from the source of the stimulus in free flight. The
investigators concluded this turning away behavior suggested an escape
function whereas a turn toward the stimulus would suggest a flight correction
function.

One of the issues with the bat-generated wind hypothesis is timing; does an
insect have enough time between detecting the bat and being captured to
perform an effective evasive maneuver? Based on the fastest behavioral
component of the P. americana aerial response (wing phase change, 38
ms) and a detection time based on a moving predator simulation (54 ms),
Ganihar predicted that a cockroach has 16 ms to evade the bat
(Ganihar et al., 1994).
Although some escape responses occur in this amount of time, i.e. crayfish
tailflip <20 ms (Krasne and Wine,
1984) and fish Mauthner-mediated C-start 14 ms
(Eaton et al., 1991), many
responses have latencies of 40 ms or more, i.e. cricket terrestrial response
87 ms (Tauber and Camhi,
1995), locust jump up to 1100 ms
(Heitler, 1974), insect flight
responses 40–240 ms, depending on insect and response type
(Nolen and Hoy, 1986;
Yager et al., 1990). However,
Ganihar et al. (Ganihar et al.,
1994) noted that their simulated predator lacked certain
characteristics of an actual attacking bat. For example, their simulated
predator moved at a constant velocity throughout an attack while a bat
decelerates before it captures a target
(Schnitzler, 1987;
Jones and Rayner, 1988). Also,
a bat flapping its wings might generate more wind than their model. Both of
these factors suggest that the amount of wind generated by an actual attacking
bat and the amount of time between detection and capture were underestimated.
Finally, instead of using the cockroach's cercal system to detect the wind
generated by the simulated approaching predator, the investigators substituted
smoke or Lycopodium spores. When the simulated predator's approach
disturbed the continuous smoke/Lycopodium stream determined the
detection time. However, the smoke/Lycopodium stream may not
accurately reflect the actual physiological cercal response of the
cockroach.

The present study addresses these issues using two methods. First, using
hot-wire anemometry, we describe and quantify the wind generated by a flying
bat attacking a target. Second, electrodes implanted in the abdomen recorded
ascending wind-evoked neural activity from an insect that served as the target
during the attack of a flying bat. Both methods provide actual measurements,
rather than predictions, to evaluate the bat-generated wind hypothesis,
via better understanding of both the wind stimulus generated by a bat
when capturing a target and the reception of this wind by the cercal
system.

This experiment used the praying mantis Parasphendale agrionina as
a target. Mantids possess an ultrasound-sensitive auditory system
(Yager and Hoy, 1989) and
perform evasive maneuvers effective for eluding bats
(Yager et al., 1990). However,
they could also benefit from evasive responses triggered by bat-generated wind
that are independent of the ultrasound-mediated responses. First, Triblehorn
and Yager (Triblehorn and Yager,
2002) demonstrated that 501-T3, the ultrasound-sensitive
interneuron likely involved in triggering mantis evasive responses, shuts down
during the last 200–300 ms of a bat attack. Furthermore, activity from
other ultrasound-sensitive neurons was not observed during this period. These
results indicate that ultrasound will not trigger any `last-ditch' evasive
responses in mantids. Second, as bats capture insects, their vocalization
emission rate increases from a very low (10–15 pulses
s–1) to very high rates (over 100 pulses
s–1). Rapid transitions from low to high rates can
potentially circumvent the mantis ultrasound defenses, either by not
triggering an evasive response or by triggering the response too close to the
capture point so that the mantis may not have enough time to perform its
evasive response (Triblehorn and Yager,
2005). In both cases, mantids could potentially benefit from a
`last-ditch' response mediated by bat-generated wind cues.

Flight room

All experiments were conducted in a carpeted, acoustically lined (Sonex I,
Illbruck, Minneapolis, MN, USA) flight room (6.4 m×7.3 m×2.5 m) at
the University of Maryland, College Park, USA, under low-light level
conditions (Fig. 1A). Two
synchronized high-speed video recorders (Kodak MotionCorders) recorded the bat
flight and capture behavior at 240 frames s–1. A 25-point
calibration frame (2.2 m×1.9 m×1.6 m; Peak Performance
Technologies, Centennial, CO, USA) placed in the center of the room was filmed
in both camera views. Both the three-dimensional position of the bat and the
target (either the mantis for physiology experiments or the anemometer probe)
as well as the distance between them were analyzed using these images and
commercial motion analysis software (Motus, Peak Performance Technologies).
The mantis was placed near the edge of the calibrated space farthest from the
release site (indicated by a filled circle on
Fig. 1A). This positioning
provided the bat with the greatest amount of time and distance to orient
itself after leaving the perch and accelerate to attack velocity before the
capture attempt.

Mantis cercal wind reception during bat attacks

Bat-detection times and distances for the mantis cercal system were
determined by using the mantis as a `biological anemometer'. Two hook
electrodes (Teflon-coated silver wires, 200 μm when bare; A-M Systems,
Carlsborg, WA, USA) were glued together (SuperGlue) and implanted in the
mantis abdomen. The electrodes recorded ascending wind-evoked activity during
bat attacks, where the mantis served as the target.

(A) Schematic of the flight room setup for the physiological experiments,
viewed from above. The gray region represents the high-speed video-system's
calibrated area for distance measurements. The dark circle marks the location
of the mantis during the experiments. (B) Photo of a trial during the
anemometer measurements of bat-generated wind. The anemometer probe sits
inside the protective cage except for the sensor region positioned about 3 cm
outside the top of the cage. In the photo, the sensor sits 24 cm below the
mealworm target (inside circle) as the flying bat approaches the target just
before capturing it. The photo is from Camera 2 (A) of the high-speed video
system.

Mantis implanted electrode procedure

Each mantis was slightly chilled until immobile. After removing the legs
and wings, the dorsal abdominal cuticle was removed, exposing the gut. The
caudal and rostral ends of the gut were tied off and the gut removed to expose
the ventral nerve cord. The abdominal connectives were cut between A1–T3
to record only ascending activity. Recording from the connectives between
A1–A2, the left connective was placed in one hook while the right
connective was placed in the other. A mineral oil–Vaseline mixture
isolated the recording from the body cavity while preventing the connectives
from drying out. After replacing the dorsal cuticle, an application of agar
(Fisher Science, Hampton, NH, USA) held the cuticle in place and prevented the
electrodes from slipping out. Placement of the indifferent wire in the
prothoracic cavity held the mantis in a `flight-like' posture. The prothoracic
connectives were removed beforehand to prevent the indifferent electrode from
recording the electrical activity from large units (such as mantis auditory
interneuron 501–T3) that would mask activity from wind-sensitive units
in the abdomen.

Recording procedure

The hook electrodes and indifferent wire were connected to a 32-gauge
braid-shielded stereo cable (Belden, Richmond, IN, USA) that served as the
tether. In the flight room, the cable tether connected to shielded coaxial
cable that carried the neural signals to the amplifier (A-M systems model
1700). Once in place, the mantis was 90–100 cm from the ceiling of the
flight room. The bat's perch was 1.5 m high and 3.37 m away from the hanging
mantis. The mantis cerci were pointed directly at the bat's perch since bats
approached and captured mantids from behind during free-flight encounters
conducted in the same flight room (J. D. Triblehorn, personal
observations).

After hanging and positioning the mantis in the flight room, but prior to
releasing the bat, neural responses to a gentle wind stimulus (the
experimenter blowing) were recorded. These wind-evoked responses were compared
to neural responses elicited during the bat attack to verify that the activity
came from wind-sensitive interneurons (based primarily on spike height).
Fig. 2 shows an example of
neural activity evoked by blowing on the cerci before the trial began with
activity recorded from the same mantis just before capture by the bat. The
similarity between some of the individual units indicates that both stimuli
evoked responses from wind-sensitive interneurons. Signal-to-noise ratios for
the largest units were 5 or better in 85% of the trials (71% of these were
over 10) and their appearance indicated the beginning of a response. A bat
detector (Pettersson model 100, Uppsala, Sweden) placed on the floor below the
mantis recorded timing information of the bat vocalizations. Neural data were
stored on DAT (Sony PCM-R500) after digitization (BioLogic DRA-400) and
analyzed offline using Superscope II (GW Instruments, Somerville, MA, USA)
after digitization (instruNet, Model 100B, Somerville, MA, USA) on a Macintosh
G3 computer.

Bat training

Since the bat's approach was critical for a successful trial, a single bat
was trained to fly consistently from a perch directly to the mantis with
little deviation. This training lasted 6 weeks, but increased the probability
that the bat would attack the mantis target (instead of the thick recording
wire) from the appropriate direction. Collection of physiological data did not
occur until the bat performed the task correctly on almost every trial. The
bat typically captured and ate the mantis off the tether. The electrodes posed
no harm to the bat since they always slipped out of the mantis and remained
attached to the wire tether.

Anemometer wind measurements

Wind generated by a flying, attacking bat was characterized using a single
axis hot-wire anemometer (Model 1700 constant temperature anemometer, Model
1210-60 hot-film probe; TSI, Inc., Shoreview, MN, USA). For one trial, the
sensor was placed 2 cm below the target to record bat-generated wind as the
bat successfully captured the mealworm target. Although the bat only tapped
the sensor when capturing the target, this contact damaged the anemometer
probe. To obtain a larger set of bat-generated wind velocity measurements, the
anemometer was placed at a distance from the target. A small cage (17
cm×13 cm×13 cm) protected the sensor by dissuading the bat from
attacking the probe instead of the mealworm target. The cage was constructed
from wire mesh (wire thickness=1 mm; mesh holes=1 cm×2 cm). The sensor
extended 5 cm beyond the top of the cage. This protective cage, however,
limited how close the sensor could be to the target and still have the bat
capture the mealworm (23 cm below the target was the closest distance the bat
would readily approach). Fig.
1B shows a photo from the high-speed video camera of the
experimental setup during one of these trials.

Implanted electrode recordings of wind-sensitive interneuron activity in
the abdominal connective of a mantis. (A) Neural activity from the mantis
recorded while blowing on the cerci (top) and just before the bat captured the
mantis (bottom). The similarity between the two responses indicates that both
traces result from wind-sensitive interneuron activity. Scale bars: 200 mV, 50
ms. (B) Same trial (shaded area in A) viewed on an expanded time scale. Scale
bars: 200 mV, 20 ms.

Results

Data of implanted electrode wind-sensitive activity elicited during bat
attacks were recorded from 18 different mantids. Two of the mantids had data
collected from two trials and the results from the two trials were averaged
for each individual. Fig. 3
contains the neural activity in the connective during the last 280 ms
preceding capture recorded for five of these trials. Each trace comes from a
different mantis and the traces span the range of detection times for all
trials. We are confident that the neural activity immediately preceding
capture is related to wind-detection because: (1) these responses resembled
the activity elicited by gently blowing on the cerci before releasing the bat
(see Materials and methods), and (2) only ascending activity was recorded
since the VNC between A1-T3 was cut (i.e. the activity did not originate from
other sensory systems rostral to the lesion, such as visual or auditory
responses). The cercal system is by far the most likely origin for ascending
activity in an isolated mantis abdomen that would generate the responses
observed just before capture illustrated in
Fig. 3.

Five examples of abdominal connective activity recorded by an implanted
electrode during the last 280 ms before a flying, attacking bat captured the
mantis. Arrows mark the beginning of the response in each trial and the
numbers state the time (distance) that the response began before capture. The
examples illustrate that wind-related activity evoked by the bat's approach
was characterized by a sudden increase in neural activity that was sustained
until the bat captured the mantis. Scale bars: 100 mV, 25 ms.

The wind-sensitive interneurons generally have a low level of spontaneous
activity in the absence of a stimulus (indicated by the activity to the left
of the arrow in each trace). Therefore, the point at which the wind-sensitive
interneurons began responding to bat-generated wind (the detection time,
marked by the arrow) was clear.

The neural responses across trials all consisted of a multiunit response
that included both large and small amplitude units. Each response continued
from the point of detection until the bat contacted and consumed the mantis
(broken line). However, detection times varied across trials as did the spike
train patterns.

On average, the mantis cercal system detected bat-generated wind during an
attack 73.9±18.8 ms (median: 77.5, range: 38–109 ms) before
contact when the bat was 27.5±7.69 cm (median: 27.5 cm, range:
14–40 cm) away. Fig. 4
shows the distribution of detection times
(Fig. 4A) and distances
(Fig. 4B) for the 18 trials.
The bat's behavior could influence when the cercal system detects the bat's
approach during an attack. One potential factor is the bat's flight velocity
as it approaches the target. In these trials, the bat's velocity in the last
200 ms before the response ranged from 292–363 cm s–1.
Within this range, there was no relationship between flight speed and
detection time or distance (velocity vs time: r=–0.04,
t(16)=–0.1763, n.s.; velocity vs distance:
r=+0.36, t(16)=1.5523, n.s.; data not shown). Another
potential factor is the changes in flight posture a bat performs to reduce its
velocity, which could potentially increase wind production suddenly, marking
the point when the mantid's cercal system detects the bat's approach. For
individuals where data from only one trial was collected, the neural responses
began within 70 ms of the time that the bat decelerated in 94% of the trials
(15 out of 16). However, the response began before the bat decelerated in 50%
of the trials (8 out of 16) and after the response began in 44% of the trials
(7 out of 16). In one trial, the response coincided with the bat's
deceleration. For the two mantids where data from two trials were collected,
responses began with 70 ms of the time the bat decelerated in all four trials.
However, for both mantids, responses occurred before deceleration in one trial
and after deceleration in the other trial.

Anemometric measures of bat-generated wind velocity

The detection times provided by the physiological recordings are crucial
for determining how much time the mantis has to perform an evasive maneuver
between detecting the bat and capture. However, it is also of interest to
quantitatively and qualitatively characterize the wind generated by a bat as
it captures a target.

Histograms of times (A) and distances (B) before capture when
wind-sensitive interneuron activity began during flying bat attacks.

The sacrifice of one anemometer probe allowed the collection of a
velocity–time waveform and peak velocity measurement at the point of the
attack. Fig. 5 shows the
anemometer voltage output leading up to the point when the bat contacted the
probe. The bat-generated wind had a high acceleration (800 cm
s–2) and reached a peak velocity of 175 cm
s–1. The fact that the wind velocity continued increasing
until the point of contact suggests that the wind may not have reached its
peak before the bat arrived at the target. The anemometer probe detected the
bat-generated wind 75 ms before contact when the bat was 18 cm away. The rest
of the anemometric data were collected with the anemometer protected and
placed near (but not at) the target (see Materials and methods).

Anemometer voltage waveforms of bat-generated wind

Bat-generated wind may vary during different phases of an attack. Normal
flight generates a certain amount of wind from the flapping wings and movement
displaces air particles in front of the bat (bow wave). Bat-generated wind
likely changes as the bat decelerates by spreading its wings and tail membrane
during a capture attempt. After capturing the target, wind follows the bat as
it leaves the capture area (slipstream).

The anemometer waveforms in Fig.
6 illustrate these changes in the bat-generated wind during
attacks. All of these examples come from trials where the anemometer was
20–30 cm directly below the target. Since the anemometer was at a
distance from the target, the capture time cannot be determined exactly from
the anemometer. However, the experiment above demonstrated that the bat's
approach was characterized by a sudden voltage increase in the anemometer
trace prior to contact. Therefore, a sudden voltage increase served as a
marker to indicate the bat's approach (indicated by the arrow for traces with
peak velocities of 56 cm s–1 or greater) for the anemometer
waveforms collected when the anemometer was near the capture point.

Anemometer output from a single trial where the probe was placed 2 cm from
the mealworm target. Contact occurred as the wind velocity continued to
increase, but possibly before the wind velocity reached its peak. Scale bars:
50 cm s–1, 50 ms.

Anemometer traces of wind generated by an attacking bat as it approaches a
mealworm target, captures it, and flies away. Since the anemometer was not
directly at the target's location, contact times (arrows) for each trace were
predicted based on the one trial when the anemometer was located at the target
(see Fig. 7). The examples
illustrate the major changes in the anemometer traces as peak velocities
increase; see text for details. The anemometer traces indicate that the bat
generates a short but strong stimulus as it approaches and captures the target
and a longer but weaker stimulus follows as the bat flies away. Scale bars: 50
cm s–1, 500 ms.

The anemometer waveforms collected during these measurements illustrate
several points. First, the duration of the bat-generated wind was long. Even
for the lowest wind peak velocity measurements (24 and 40 cm
s–1 peak velocity traces), the anemometer detected
bat-generated wind for at least 1 s. Second, during this period, the magnitude
of the bat-generated wind fluctuated. Weakly generated winds (24 and 40 cm
s–1 peak velocity) contained only slow fluctuations in
velocity and lacked a distinct peak. A distinct peak in the velocity appeared
in traces with 50–60 cm s–1 peak velocities, but was
not prominent until around 80 cm s–1 peak velocity. Waveforms
with peak velocities greater than or equal to 80 cm s–1
contained both rapid fluctuations in velocity and the slower fluctuations
observed in weaker wind profiles. Third, for stronger wind measurements, there
was a rapid transition from a `no wind' condition (zero velocity) to a strong
wind condition. In fact, the early portion of the profile usually contained
the strongest component for trials measuring peak wind velocities over 80 cm
s–1. In summary, these anemometer measurements indicated that
attacking bats generated wind detectable at a single point near the target
over a long period and that the strongest portion of the stimulus (both in
terms of velocity and acceleration) occurred at the beginning of the generated
wind stimulus.

A large sample of peak velocity measurements of bat-generated wind were
collected with the anemometer probe placed close to the target (but not
directly at the target). Fig.
7A shows the peak velocities of bat-generated wind for 94 samples
as a function of the anemometer distance. The highest wind velocity recorded
was 326 cm s–1 (28 cm from the target) while the lowest was 6
cm s–1 (27 cm from target). The distance between the
anemometer and the target accounts for some of the variation seen in the data.
However, even when the anemometer was between 18–30 cm from the target
(representing 77% of the trials; shaded area in
Fig. 7A), bat-generated wind
velocity measurements still exhibited substantial variation unrelated to
distance. In 38.5% of the trials, the measured peak wind velocities were below
50 cm s–1. In 31% of the trials, bat-generated wind contained
peak velocities between 50–100 cm s–1 and peak
velocities exceeded 100 cm s–1 in the remaining 30.5% of the
trials.

In the other 23% of the trials, the probe was 30–50 cm away from the
target. In these cases, the anemometer probe was below the target, but off to
the side and not directly below. This setup simulated situations where a
mantis evades capture by performing a power dive and determined how much wind
would reach the mantis as the bat flies by. Since the mantis auditory system
lacks the ability to localize sound (Yager
and Hoy, 1989; Yager et al.,
1990), bat-generated wind could indicate the location of the bat
to the mantis. Mantids could then potentially incorporate this new sensory
information into the power dive response to direct the mantis further from the
bat's location and increase its chances of survival. At these distances, the
anemometer probe still detected bat-generated wind, but the peak velocities
were lower, ranging between 20–60 cm s–1 peak velocity
(Fig. 7A).

(A) Peak wind velocities measured by the anemometer at distances
20–60 cm from the target (N=94). The anemometer was within
18–30 cm of the target in 77% of the trials. The shaded boxes indicate
the percentage of measured peak wind velocities that fell between 0–49,
50–99 and >100 cm s–1 for the data collected with
the anemometer within 20–30 cm of the target only. (B) Distribution of
peak accelerations for 66 of the trials in A. Peak acceleration was calculated
using the onset time of the anemometer detecting the bat-generated wind to the
time of the peak velocity. (C) Comparison of peak velocity and peak
acceleration for each trial in B. For bat-generated wind, peak velocity and
peak acceleration were closely related, with peak acceleration increasing
exponentially with increasing peak velocity. The equation for the exponential
best fit line is
f(x)=4.980398×10–1*exp(4.814839×10–2*x).

Discussion

The possibility that insects possessing cercal systems may detect wind
generated by an attacking bat and use this cue to mediate flight evasive
responses is intriguing for both deaf and hearing insects. For deaf insects,
in the absence of visual input, this cue may be the only indication that an
attack is imminent. For insects sensitive to bat echolocation calls,
bat-generated wind detection may act either in conjunction with the auditory
system or as a backup response in case the auditory defense fails. The
usefulness of this cue for insects depends on how much wind a bat generates as
it approaches a target and whether the insect detects the wind early enough to
perform an effective evasive response.

Physiological recordings of wind-sensitive interneuron activity during bat
attacks provide good measurements of detection times and possess three
distinct advantages over the hot-wire anemometric measurements: (1) they
provide the most accurate information about bat-generated wind (although
uncalibrated regarding the wind's magnitude) since the `sensor' serves as the
target, (2) they provide the most relevant information for evaluating the
possible success of wind-evoked responses (by providing detection time and
distances) since the natural receiver detects the stimulus from the natural
producer, and (3) they allow the collection of multiple trials without risking
the bat or expensive equipment.

Hot-wire anemometry provided calibrated measurements of bat-generated wind
not available in the physiological recordings. The measurements include the
magnitude of the wind generated as well as how the generated wind pattern
changes at a single point over a period of time.

The nature of wind generated by an attacking bat and its
measurement

The wind stimulus created by an approaching bat should be very complex and
will vary with the angle of approach, initial bat flight speed, wing stroke
angle and frequency, rate and timing of deceleration, and the effective area
of the bat facing the anemometer probe. The air motion will be turbulent,
hence varying unpredictably in velocity and direction. The complexity and
variability of the wind produced by an attacking bat makes the measurements of
the bat-generated wind using a single-axis hot-wire anemometer probe with a 2
mm sensor surface difficult. In addition to the factors mentioned above, the
probe only samples a small region of the entire wind stimulus. However, these
are the same difficulties faced by an insect's cercal system when detecting
the wind from an approaching bat. P. agrionina's cerci occupy as much
space as the anemometer probe and, therefore, sample only a small portion of
the bat-generated wind. Furthermore, cercal systems are directionally
sensitive (Palka et al., 1977;
Dagan and Camhi, 1979;
Westin, 1979), similar to the
anemometer probe. Therefore, certain characteristics of the data collected
using the anemometer, such as the large variation in the data when the probe
was within 20–30 cm of the target, fully represent and describe the
situation involving the cercal system.

Despite the predicted and observed variation in the wind stimulus, the
anemometer data consistently show an initial peak with a high acceleration and
the maximum velocity for the trial. The early appearance of these components
represents the insect's first indication of a bat's approach based solely on
wind. Furthermore, the anemometer data unquestionably underestimate the
maximum velocity and probably the acceleration, since the probe was not
directly at the target for the majority of the measurements. Variation in
maximum velocity may be relatively unimportant to the insect, because even our
underestimates are orders of magnitude above detection threshold for the
cercal system (Triblehorn,
1997). The physiological recordings confirm that such a
bat-generated wind stimulus sufficiently excites several wind-sensitive
interneurons.

Wind stimulus acceleration, not peak velocity, may be the more important
parameter for triggering an evasive response. This is the case in
cercal-mediated terrestrial responses and is important for the animal
distinguishing between wind generated from a predator (with high
accelerations) that trigger an escape response vs changes in ambient
wind conditions (with low accelerations though potentially high velocities)
that do not trigger escape responses
(Plummer and Camhi, 1981). For
cercal-mediated aerial responses, the acceleration component may be even more
important since flight provides continuous wind stimulation on the cerci with
relatively high velocities (around 180 cm s–1 in mantids and
cockroaches) but with low accelerations during stable flight.

In cockroaches, wind stimuli with high accelerations (60 cm
s–2 or higher) evoke escape running, while stimuli with
accelerations around 30 cm s–2 evoke a pause in walking
(Camhi and Nolen, 1981). The
peak acceleration of the bat-generated wind often exceeded 200 cm
s–2, more than three times the acceleration necessary to
evoke escape running in cockroaches. The peak accelerations measured in our
experiments using stationary targets (both the anemometer and mantis
physiological preparation) may be overestimates compared to the natural
situation where the mantis would be flying. As mentioned previously, however,
the anemometer probe was not located directly at the target and, therefore,
likely underestimates the actual peak acceleration. Even so, the peak
accelerations measured were very high (a large proportion were >200 cm
s–2). Furthermore, the peak acceleration was 800 cm
s–2 for the one trial where the anemometer was located at the
target.

Evaluating the bat-generated wind hypothesis: is there enough time to
escape?

The physiology results showed that the mantis cercal system detected the
bat an average of 74 ms before contact, when the bat was 27.5 cm away. Since
the electrodes were implanted to record from the connectives between
A1–A2, these detection times actually incorporate the detection of the
bat-generated wind by cercal hairs, afferent neural conduction, processing and
information transfer to ascending interneurons in the terminal ganglion, and
conduction time to reach the electrodes. These results correlate well with the
detection time measured by the anemometer placed within 2 cm of the target (75
ms), but not as well with the detection distance measurement (18 cm). Still,
18 cm is within the range of detection distances from the physiological
measurements. It is interesting that this detection time matches the cockroach
cercal system's detection time of the wind produced by the tongue strike of an
attacking toad, a natural predator that evokes the terrestrial wind-evoked
response (Camhi et al., 1978).
Based on the wind cue alone, cockroaches escaped 55% of the time (47%
advantage over cockroaches with cerci covered and unable to detect wind).
Therefore, it is not unreasonable that using the wind cue may improve the
mantid's odds at escaping when the auditory defense fails.

Comparisons between the bat's flight velocity at detection indicated that
this parameter was not correlated to either an earlier detection time or
distance. However, the tail-flip used by a bat to capture an insect and
transfer it to its mouth begins about 65±15 ms before capture
(N=10; K. Ghose, personal communication). This value is close to the
74 ms average detection time, suggesting that the tail-flip may be a major
contributor to the bat-generated wind that the cercal system detects.

Based on their predator simulations, Ganihar et al.
(Ganihar et al., 1994)
estimated that an insect could detect an approaching bat predator 54 ms before
contact (40–68 msrange). The current assessment of average detection
time (74 ms) based on physiological recordings allows the insect 20 ms more to
escape. The new estimate allows the cockroach a total of 36 ms to evade the
bat (based on the cockroach's 38 ms behavioral response latency;
Ganihar et al., 1994). Even
with the increase in the detection time from Ganihar's estimate, it still
seems unlikely that enough time exists for a cockroach (or any insect) to
detect the bat and perform a response that will cause a bat to completely miss
it. However, there may be enough time for the insect to alter its position
just before capture. Although researchers suggest that echolocating bats
update the target location on an echo-by-echo basis
(Masters et al., 1985;
Masters, 1988), others suggest
that bats plan their capture attempt early by predicting the location of the
insect at the point of capture (Wilson and
Moss, 2002; Ghose et al., in
press). If bats do plan their capture attempt in advance, any
wind-evoked behavior altering the insect's flight would result in a difference
between the bat's prediction and the insect's actual location. This deviation
could cause the bat to mishandle the insect (i.e. drop it) since the bat would
not be able to adjust to the unpredicted change. Earlier wind detection would
give the insect more time to respond, resulting in a larger discrepancy
between the bat's prediction and the mantid's actual location. Therefore,
successful wind-evoked insect evasive maneuvers would likely not cause the bat
to completely miss the insect, but cause the bat to mishandle the target
during capture by tipping the insect with its wing or tail membrane and give
it one final chance to avoid the bat. Such late changes in the insect's
trajectory could also pose problems if bats do update a target's location on
an echo-by-echo basis. Although the bat may detect the change, it may not be
able to act on this information and adjust its capture attempt to compensate
for the alteration of the insect's location in such a short time.

In staged free-flight encounters between mantids and bats, we have observed
bats making contact with deafened mantids (thus eliminating the
ultrasound-triggered evasive response) and dropping them
(Triblehorn, 2003). These
mantids were not bitten, so they were not dropped after the bat transferred
the mantis to its mouth and began eating. Although no behavioral responses
from the mantids were observed, such responses would occur very close to the
time of capture (within 100 ms) and would not likely be seen by the observers
(though the sound of the bat making contact with the mantis was evident).
Although such observations could be coincidental mistakes in the bat handling
the mantis, it is also possible that these dropped, deafened mantids
successfully detected and responded to wind from the approaching bat, enabling
them to survive the attack.

The fragility of the implanted mantis preparation and the nature of the
experiment as a whole prevented establishing, without a doubt, that the
activity recorded during the bat attacks are from ascending wind-sensitive
interneurons. However, in addition to the data presented here (see
Fig. 2 and Results), data from
other studies conducted in our laboratory indicate that this is most likely
the case. First, mantids do possess filiform hairs that are sensitive to wind
stimulation, and removing and/or shortening these hairs increases wind
detection thresholds measured by recording from the cercal nerve
(Triblehorn, 1997). Second,
extracellular recordings from abdominal connectives in P. agrionina
reveal that wind puffs of varying velocities elicit responses from multiple
ascending interneurons (Triblehorn,
2003). These interneurons vary in spike height and firing
properties (i.e. find both phasic and tonic units). Furthermore, covering the
cerci with Vaseline eliminated these wind-elicited responses (J. D.
Triblehorn, personal observation). Finally, in the current study, with the
exception of testing the mantis preparation by blowing on the cerci, neural
responses resembling those elicited by the approach of the bat did not occur,
either spontaneously or as a result of the experimenter's actions while
preparing for the trial.

Using bat-generated wind during escapes

Anemometer measurements show that bats can generate wind detectable at
least 50 cm away from the target. Mantids are unable to localize sound and
cannot determine the approaching direction of the attacking bat
(Yager and Hoy, 1989;
Yager and May, 1990). When the
bat is close, the mantis executes a power dive to evade the bat and does not
necessarily need to know the bat's approach direction. However, the wind
created as the bat passes the diving mantis could serve as a cue to the bat's
location. The mantis could incorporate this new sensory information into its
power dive, giving the dive a directional component allowing the mantis to not
only escape from the bat's initial attack, but also enable it to avoid a
second attack.

The use of bat-generated wind in these scenarios only seems reasonable if
the mantis is within 30 cm of the bat (see
Fig. 7). Within this distance,
peak velocities exceed 100 cm s–1 in 30.5% of the trials.
However, beyond this range, peak velocities rarely exceeded 60 cm
s–1. Wind puffs at 60 cm s–1 failed to
elicit strong behavioral responses in flying cockroaches
(Ganihar et al., 1994) or
significant changes in P. agrionina wind-sensitive interneuron
activity in the presence of a flight-simulated headwind
(Triblehorn, 2003). In the
latter experiments, the flight-simulated headwind was set to 180 cm
s–1 (equal to the mantis stable flight velocity). When
executing a power dive, mantids double their flight velocity, which would
potentially make it more difficult for the cercal system to detect the
bat-generated wind. However, this increase is not instantaneous and mantids
should be too far from the bat by the time they reach this velocity for the
bat-generated wind to be a factor. However, even when flying at 180 cm
s–1 at the initiation of a power dive, it seems unlikely that
mantids should rely on bat-generated wind for determining the direction of the
attack and incorporate this information into their response.

Candidates for putative wind-mediated evasive responses

Insects that would benefit from wind-mediated evasive response to avoid bat
predation are those that possess cerci and fly nocturnally. Despite the
prevalence of either of the characteristics across insect species, those that
possess both are in the minority. The majority of insects that possess cerci
are wingless and do not fly. On the other hand, all of the holometabolous
(complete metamorphosis) insects (such as Diptera, e.g. flies; Lepidotera,
e.g. moths and butterflies; Homoptera, e.g. cicadas and aphids; Hymenoptera,
e.g. bees, ants and wasps; and Coleoptera, e.g. beetles) as well as Hemiptera
(e.g. the true bugs) and Neuroptera (e.g. lacewings and caddis flies) have
lost their cerci but have the ability to fly. The main candidates for putative
wind-mediated evasive responses are members of Dictyoptera (e.g. mantids and
cockroaches) and Orthoptera (e.g. crickets, locusts, grasshoppers,
katydids).

Insect `last chance' bat avoidance responses

Late `last-chance' behaviors, mediated by ultrasound-sensitive auditory
systems, have been described in several insects. Green lacewings passively
fall in response to bat vocalizations emitted at low rates, but as the bat
closes in on the capture, the higher emission rates trigger `wing-flips' in
the lacewing that abruptly slow its descent
(Miller and Olesen, 1979).
`Wing-flips' occur about 50–100 ms before capture and this unexpected
alteration in the lacewing's descent occurring late in the bat's capture
attempt causes the bat to miss the insect 70% of the time. Arctiid moths
produce ultrasonic `clicks' that deter bats from capturing the insects as an
acoustic aposematic signal (Dunning,
1968; Dunning et al.,
1992; Hristov and Conner,
2005), by startling the bat
(Möhl and Miller, 1976)
or `jamming' its ability to echolocate
(Fullard et al., 1979). High
bat emission rates trigger arctiid moth `clicks' during the late stage of the
bat attack (Fullard, 1984;
Fullard et al., 1994) and
begin between 142–270 ms before capture, depending on stimulus level.
Acharya and Fenton (Acharya and Fenton,
1992) showed that bats rarely contact `clicking' moths even though
the arctiids show little, if any, evasive behaviors during the attack.

These `last-chance' maneuvers do not require significant alterations in
flight trajectories to be effective, which likely contributes to their high
success rate. The lacewing passive fall relies on gravity and the `wing-flip'
simply alters the velocity of this fall, not its trajectory. Arctiid moth
`clicks' effectively protect the insect without any alteration in its flight
path. However, for the putative wind response, success potentially relies on
how much the insect can alter its trajectory from the bat's prediction during
its capture attempt. This requires time to initiate the response internally
(neuromuscular response) and for the response to take effect (resulting in a
change in flight path). Given the short amount of time for this to occur, the
success rate will likely be much lower than the other `last chance' maneuvers.
Still, very low survival advantages can be very important evolutionarily
(Endler, 1986). Since the
response makes use of the neural, muscular and behavioral elements already
present and in use during normal flight, the cost to the insect is minimal
while providing a slim chance to survive the encounter, which is better than
no chance.

ACKNOWLEDGEMENTS

The authors would like to thank Dr Cynthia F. Moss for allowing us to use
the flight as well as the video recording and analysis equipment. The authors
would also like to thank Melinda Byrns and Sachin Vaidya for their help in the
laboratory. Finally, we would like to thank the two anonymous reviewers for
their helpful comments that improved the manuscript. This work was supported
by NSF grant no. IBN9808859 (D.D.Y.) and NIMH (NRSA) grant no. F31MH12025
(J.D.T.).

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